In cementless orthopedic procedures robust biologic ingrowth may be a key element to long-term implant stability and performance. Biologic ingrowth may require sufficient stability of the implant with respect to adjacent bones and/or tissues, particularly during the first 6-8 months after implantation. During this time, bone growth onto a roughened surface or into a porous surface may only occur if the implant is held stable with respect to the bone. Ideally, in order to achieve desired stability, motion of the implant relative to the bone should be less than 150 microns particularly in the liftoff (i.e. normal) and sheer (i.e. bi-directional) directions with respect to the plane of the porous or roughened surface.
Implant manufacturers routinely utilize a variety of design features to attempt to provide a press-fit that aids in limiting the movement of the implant relative to surrounding anatomical structures. Fixation features such as pegs and keels, for example, generally include designed surface textures to increase the coefficient of friction between the implant and the surrounding anatomical structures. These implant fixation features may result in increasing success of cementless implants. However, bone quality including bone density, porosity, and elastic modulus, for example, varies from patient to patient and also varies by location within a particular patient. Variability in bone quality at the location which bone contacts fixation features results in variable effect of fixation and subsequently may result in deviations of the implant stability from the design intent.